Question

For a reaction
${H_2} + C{l_2} \to 2HCl$
Rate= K
1. Write the order and molecularity of this reaction
2. Write the unit of K

Answer 1

The order of a reaction is dependent on the power of the concentration of the reactants involved in the reaction. Molecularity is determined by the number of molecules or ions involved in the reaction.

Complete answer:
1. The Order of reaction is the relationship between the rate of a chemical reaction and the concentration of the reactants participating in it. The expression for the rate of order of a reaction is given by:
$r = k{[A]^x}{[B]^y}$ .
The order of a reaction can be zero order, first order, second order and pseudo order.
In the aforesaid equation the rate of the reaction is independent of the concentration of reactants, so it is a zero order reaction.
Similarly, the molecularity of a reaction represents the number of atoms, molecules, or ions that interact with each other for the chemical reaction to occur within a limited time frame.
In the above reaction two molecules of hydrogen interact with two molecules of chlorine to form two molecules of hydrogen chloride. Therefore,the molecularity of the aforesaid reaction is 2.

2. The rate of a reaction is denoted by ‘K’.
Unit of K or for the rate of the reaction is : $mol{L^{ - 1}}{s^{ - 1}}$

Note:
Order of a reaction is determined experimentally. It is either the whole number or a fraction, whereas molecularity is determined by the balance of a rate of a reaction. It is always a whole number. Order of a reaction is power of concentration of reactants whereas the molecularity of a reaction is determined by rate determining step of a reaction.